The call waiting ("CW") feature available on many telephone operating systems allows the telephone operating system to notify a user currently communicating with a second party over a telephone line that a third party is attempting to call the subscriber. Typically, the system notifies the user of the third party's call by using a special CW tone. The subscriber has the option to put the second party on hold to receive the third party's call or ignore the CW tone.
The caller identification ("CID") feature available on some telephone operating systems allows the telephone operating system to transmit a caller's telephone number as a digitally coded signal to the user's telephone or telephone with adjunct (hereinafter referred to as a "CPE" for customer premises equipment). The user's CPE has circuitry to receive and decode the digital signal and display the caller's telephone number. This allows the user to answer or ignore the call based on the caller's telephone number.
Some CPEs allow for using the CID and CW features together in a CIDCW system, whereby the user's telephone set displays the third party's caller information (e.g. telephone number, caller's name, or both) while the user is communicating with the second party. These conventional systems generally detect whether an extension CPE is off-hook (which typically means that a user is using this extension CPE) and terminate the CID feature upon such detection. These CIDCW systems do not allow for simultaneously using multiple conventional CIDCW CPEs on the same telephone line for at least the following reasons: the circuitry of the conventional CIDCW CPEs tests for other CPEs being off-hook and shuts off the CIDCW capability in such cases; the user(s) would hear the CID information being transmitted from the central office (the CID information signal is typically irritating for the user(s) to hear); any voice signals from the user(s) would interfere with the handshaking signals used in transmitting the CID information; and the conventional CIDCW CPEs would cause contention problems when each CIDCW CPE attempts to transmit an acknowledgement signal in response to a signal from the operating system indicating a third party is trying to call.